In recent years, the occasion of mailing various kinds of documents on privacy of individuals, e.g., bills, notices for payment, receipts and notices for expiration which relate, e.g., to taxes, annuities, bonds, deposits and savings, credits, telegram and telephone charges, gas charge, water charge, electrical charge and so on, has markedly increased in number.
In mailing documents regarding privacy of each individual, sealed letters have so far been used from the standpoint of protecting privacy. However, mailing documents by sealed letter has the demerit of high price, compared with mailing documents by postcard.
Under these circumstances, various types of postcards capable of keeping the contents of a private message secret have been proposed in recent years.
As one of these proposals, instanced a paper seal is applied to the surface of a conventional postcard, on which letters and figures are recorded, having a peel-apart possibility in order to temporarily veil the contents of the record. In the paper seal of this kind, a weak tackifier is used to form its adhesive face, so it is necessary to laminate a release paper for the protection of the weak tackifier layer. The use of a release paper is disadvantageous in that it causes an increase in cost, and is attended with a trouble of its disposal. Moreover, the paper seal-applied postcard has the defect that part of the letters and the like written on a postcard is stripped off and transferred to the weak tackifier layer in peeling the paper seal off the postcard to make it hard to read. What is worse, the paper seal can be delaminated and relaminated so as not to leave traces of taking off the seal. Accordingly, the paper seal suffers another defect in that it is inferior for protecting against invasion of privacy.
As another material, there can be cited a label of the type which is a sheet lined with aluminum foil, applied to the letters-written side of a postcard in order to screen the message on the postcard, and designed so that breaking the seal becomes possible only when the sheet is cut open along perforations hemming round the postcard.
However, this label is expensive because it is a laminate having a composite structure and, what is worse, it is hard to break into pieces and to destroy by fire. Therefore, it suffers from a disposal problem.
In addition, the foregoing paper seal and label can be used only for veiling the recorded information. Accordingly, there has been no other means of mailing a great deal of information but to adopt the procedure of mailing in sealed covers which is higher in price.
In order to solve the above-described problems, we have previously proposed a transparent heat-sensitive adhesive sheet having adhesiveness on both sides (Jitsuyo Kokai No. 02-25546, wherein the term "Jitsuyo Kokai" used means to "unexamined published Japanese utility model application"). By the use of this sheet, the information-recorded faces of two sheets of postcard-size recording material can be bonded together in a condition whereby they can be peeled apart from each other when necessary.
However, it is essential for the process of manufacturing the foregoing transparent heat-sensitive adhesive sheet to involve a step of providing a thermoplastic resin layer on one side of a transparent plastic sheet in a releasable condition using a fusion extrusion technique. This step makes the manufacturing process complicated. In addition, not only the transparent plastic sheet but also the thermoplastic resin layer formed by fusion extrusion are hard to break into pieces by hand, so it is necessary to dispose of the information-recorded material using a special disposer such as a shredder from the standpoint of keeping the information secret. Disposal of this sort is inconvenient for private mails. Moreover, a transparent material is used as the support of said transparent adhesive sheet so that the information recorded on the face where the support remains can be read since the support is left on one of the information-recorded faces when the information-recorded material is torn open. This causes according to the quality of the recording material a serious situation that the secrecy of the information recorded inside the recording material is not preserved to a satisfactory extent.
On our further examination, it has been found that an entirely satisfactory result can be obtained by using a heat-sensitive adhesive sheet which has a support and a synthetic resin layer provided on each side thereof in a condition that it can be peeled apart therefrom if needed, with the synthetic resin layer having an ability to adhere to an information-recorded face in a substantially unreleasable condition when heat is applied thereto, thus achieving this invention.